[FIX] header_size: JS sparse array problem

[dhrp-odoo]

Description:

JavaScript methods can behave inconsistently with sparse arrays, often ignoring empty indexes. For example, Array.prototype.filter() skips empty indexes.

This commit resolves the issue by explicitly assigning undefined to all indexes when creating the sheet, ensuring that the insertion and removal of rows and columns work as expected.

Task: : 4074438

review checklist

• [ ] feature is organized in plugin, or UI components
• [ ] support of duplicate sheet (deep copy)
• [ ] in model/core: ranges are Range object, and can be adapted (adaptRanges)
• [ ] in model/UI: ranges are strings (to show the user)
• [ ] undo-able commands (uses this.history.update)
• [ ] multiuser-able commands (has inverse commands and transformations where needed)
• [ ] new/updated/removed commands are documented
• [ ] exportable in excel
• [ ] translations (_t("qmsdf %s", abc))
• [ ] unit tested
• [ ] clean commented code
• [ ] track breaking changes
• [ ] doc is rebuild (npm run doc)
• [ ] status is correct in Odoo